The present invention relates to a method of obtaining a representation of an electrical circuit suitable for time-domain simulation, and further relates to an apparatus comprising a circuit modeller for obtaining such a representation. The electrical circuit comprises a physical structure, which is modelled using electromagnetic field analysis, and also comprises a remainder circuit of circuit components which are interconnected with the physical structure.
Use of computer circuit simulation tools to predict the performance of electrical circuits, comprising a plurality of circuit components such as resistors, capacitors and transistors, is widespread in the field of electrical circuit design, resulting in savings in both time and design costs. The Philips PSTAR.TM. circuit simulator is one such tool, capable of various types of analysis including frequency-domain and time-domain.
Some circuit components, such as transistors and diodes, exhibit a non-linear relationship between the current passing through them and the voltage developed across their terminals. For many purposes such devices can be modelled by first establishing an operating point, from a consideration of the DC voltages applied to them, and then treating them as linear devices. This is known as small signal analysis. It is a computationally efficient model, and is the method used in frequency-domain analysis by a circuit simulator.
However, such analysis does not predict any effects caused directly by the non-linear behaviour of circuit components, such as the generation of harmonics and distortion. Such effects are not modelled in a frequency-domain analysis, but are modelled in a time-domain analysis which takes into account any non-linear relationship between current and voltage in components. Time-domain analysis requires considerably more computational resources than frequency-domain analysis, as well as more care to ensure that the results of the analysis are meaningful.
In addition to the circuit components an electrical circuit requires a physical structure to support the components and to provide electrical connections between them, for example a Printed Circuit Board (PCB) and its associated conductor pattern. It is often the case that this physical structure also affects the performance of the electrical circuit, for example through the resistance of an interconnection or mutual inductance between two interconnections. The physical structure can also be used to implement circuit components, such as inductors and capacitors, and may have additional features such as metallic enclosures for shielding purposes.
Modelling the effect of the physical structure on the performance of the electrical circuit can be done by using an electromagnetic simulator, which can determine charge and current densities, and the resulting electric and magnetic fields, throughout the structure. Preferably, the results of this analysis should be in the form of a lumped component equivalent circuit for the structure so that the electrical performance of the combination of the circuit components and the physical structure can be determined using a circuit simulator.
An example of such an electromagnetic simulator is available under the Trade Mark FASTERIX from product divisions and associated companies of Philips Electronics N. V. of Eindhoven, the Netherlands. FASTERIX.TM. is disclosed in the paper "FASTERIX, an environment for PCB simulation" by R du Cloux et al in the published papers of the 10th International Zurich Symposium on Electromagnetic Compatibility, Mar. 9-11, 1993, pp.213-218. Examples of its use are disclosed in this paper, as well as in the papers "Reduced Equivalent Circuit Model for PCB" by R F Milsom et al, in Philips Journal of Research, Vol 48 pp.9-35, 1994, and "EMC simulations and measurements" by R du Cloux et al in the published papers of the 11th International Zurich Symposium on Electromagnetic Compatibility, Mar. 7-9, 1995, pp. 185-190, and published European Patent Application EP-A-0 615 204 (and its equivalent granted United States patent number U.S. Pat. No. -A-5 625 578, our reference PHN 14402). The whole contents of all these three papers and EP-A-0 615 204 are hereby incorporated herein as reference material.
The electromagnetic simulator embodiment described in EP-A-0 615 204 and the said three papers is suitable for simulating a PCB layout, which has one or more conductor patterns of a uniform conductivity and thickness which may be on one or more layers on or within an insulating substrate of uniform dielectric constant, and in which electromagnetic energy loss in the conductor patterns and in the dielectric substrate is low. The standard model in the FASTERIX.TM. simulator is, however, very inaccurate when used directly for simulating a so-called "passive integration IC", which is an integrated circuit (IC) having thin-film passive circuit components. The predominant reasons for this inaccuracy are that energy losses are greater than in a PCB, the loss mechanisms are different, and the layer thicknesses differ much more widely.
Modifications to enable an electromagnetic simulator such as FASTERIX.TM. to simulate passive integration ICs are disclosed in our co-pending, unpublished PCT Application IB98/00400 (PHB 34150). The whole contents of this patent application is hereby incorporated herein as reference material.
In principle an equivalent circuit model of a physical structure can be used for time-domain simulations. Since it only includes passive, linear components a single frequency input results in a single frequency output. In practice there are considerable problems because the equivalent circuit may only be valid over a limited range of frequencies, and may contain components whose value varies with frequency. As a result it has not so far been possible to perform accurate time-domain simulations for a remainder circuit including active non-linear circuit components, where the power source introduces a DC component and the non-linearity introduces a whole spectrum of frequencies even for a single frequency AC input.
For example, two of the equivalent circuit models of a physical structure that can be generated by the FASTERIX.TM. simulator are known as "high-frequency" and "full-frequency". The high-frequency model is the simpler of the two and has been used successfully in time-domain simulations, but it is not correct at DC and cannot therefore be used for time-domain simulations including active non-linear components.
The full-frequency model is valid for all frequencies from DC to a specified upper limit. However, it is considerably more complex than the high-frequency model and often gives very poor results when used for time-domain simulations. One reason for this is that the additional complexity, required to obtain a valid model, gives rise to numerical instability in the circuit simulator.